1. Field of the Invention
This invention relates to a frequency demodulating device and more particularly to a device using a low frequency carrier wave in frequency demodulating a frequency modulated signal.
2. Description of the Related Art
Apparatuses for recording or reproducing (or transmitting) signals by frequency modulating them using low frequency carrier waves, such as video tape recorders, have been arranged to demodulate for reproduction the frequency modulated signals, for example, by a pulse counter demodulating method in which the frequency modulated wave is demodulated with a pulse counter after it is twice stepped up through full-wave differentiation or by a quadrature demodulating method.
FIG. 1 of the accompanying drawings shows by way of example the arrangement of the conventional frequency demodulating device of a video tape recorder (hereinafter referred to as VTR). The conventional device has been arranged as follows: A frequency modulated signal is reproduced from a magnetic tape 10 by means of a magnetic head 12. The reproduced signal which has the lowest frequency fc of a carrier is supplied to a limiter 16. The limiter 16 restricts the signal to a given amplitude. A full-wave differentiation circuit 18 full-wave differentiates the output of the limiter 16. The circuit 18 then detects the rise and fall edges of the output of the limiter 16. The full-wave differentiation circuit 18 produces a frequency modulated signal with a carrier frequency of 2 fc. The output of the circuit 18 is applied to a monostable multivibrator 20 (hereinafter referred to as MM). The MM 20 converts the frequency modulated signal into a pulse train of a given pulse width. The output of the MM 20 is supplied to a low-pass filter 22 (hereinafter referred to as LPF). The LPF 22 produces a demodulated signal.
In the frequency demodulating device which is arranged in this manner, a higher portion of a lower side wave enters into the band of the demodulated signal to cause a distortion (or a moire in the case of a video signal). More specifically, assuming that the signal to be recorded is a video signal of a frequency band as indicated by a broken line in FIG. 2(a) and that the highest frequency of the band is fa, the frequency occupying band of a frequency modulated signal obtained by frequency modulating the above stated recording signal becomes as shown in FIG. 2(b). In FIG. 2(b), a reference symbol fc denotes the lowest frequency of the carrier used for the frequency modulation and indicates the lowest frequency within an FM deviation. The frequency of a first lower-side wave J1 is fc - fa. A signal obtained by demodulating the frequency modulated signal of FIG. 2(b) with a known ordinary 2-step-up pulse counter demodulator which is as shown in FIG. 1 and the frequency spectrum distribution of the demodulated signal are as shown in FIG. 2(c). In the demodulated signal, the frequency modulation carrier frequency is increased to 2 fc by a 2-step-up action. As a result, there arise first and second lower-side waves J1 and J2. The frequency of the first lower-side wave J1 is 2 fc - fa, and the frequency of the second lower-side wave J2 is 2 fc - 2 fa. If the frequency of 2 fc - 2 fa is not higher than the frequency fa, or if the frequency fc is somewhat higher than a frequency 3 fa/2, the second lower-side wave J2 would enter the band of the demodulated signal, as shown in FIG. 2(c). Then, in the case of a video signal, this causes a moire. Further, with the frequency modulated signal 2-step-up multiplied, the deviation doubles to double thereby the energy of the second lower-side wave J2. Therefore, this makes the moire more conspicuous. To avoid this, if the frequency modulation is arranged to be carried out with the frequency fc set at a value higher than 3 fa/2, the highest frequency fa of the video signal or the like would become too high for a frequency band recordable by a magnetic recording and/or reproducing apparatus.
In view of this problem, a 4-step-up demodulating device has been proposed to permit frequency modulation at fc&lt;3 fa/2, fc&gt;fa (Japanese Laid-Open Patent Application No. SHO 61-8771). FIGS. 3(a) to 3(c) show frequency allocation made by the frequency modulating and demodulating method of this 4-step-up demodulating device. FIG. 3(a) shows the frequency spectrum distribution of a signal to be frequency modulated. FIG. 3(b) shows the frequency spectrum distribution band of the frequency modulated wave. FIG. 3(c) shows the frequency spectrum distribution obtained with the frequency modulated wave of FIG. 3(b) stepped up by four times. Through the 4-stepping up process, the frequency modulation carrier frequency becomes 4 fc. There arise a first lower-side wave J1, a second lower-side wave J2 and a third lower-side wave J3. The frequency of the first lower-side wave J1 is 4 fc - fa. That of the second lower-side wave J2 is 4 fc - 2 fa. That of the third lower-side wave J3 is 4 fc - 3 fa. In order to prevent the third lower-side wave J3 from entering the band of a demodulated signal, indicated by a broken line in FIG. 3(c), these frequencies are arranged to be in the following relation: fa&lt;4 fc - 3 fa. Therefore, fc&gt;fa. In other words, in accordance with this 4-step-up demodulation method, the frequency modulation carrier frequency fc which is the lower limit frequency of FM deviation as shown in FIG. 3(b) is arranged to be higher than the highest frequency fa of information to be recorded or transmitted. This arrangement not only improves the S/N ratio of reproduced signal but also permits use of a narrow transmission route.
FIG. 4 shows by way of example the arrangement of a VTR which is of the kind arranged to frequency modulate a luminance signal, to convert a carrier chrominance signal to a lower band and to record them in a multiplexing manner. The VTR has the above stated 4-step-up demodulating device employed in the luminance signal reproducing system thereof. A reference numeral 100 denotes the 4-step-up demodulating device. A frequency modulated luminance signal which is reproduced from a magnetic tape 10 by a magnetic head 12 is supplied to a transversal type phase shifter which is formed jointly by a delay circuit 116, a subtracter 118 and an adder 120.
The operating principle of the transversal type phase shifter is briefly described as follows: Assuming that a signal supplied to the delay circuit 116, the subtracter 118 and the adder 120 is expressed as Fa, the output of the delay circuit as Fb, that of the subtracter 118 as Fx, that of the adder 120 as Fy, an angular frequency as .omega. and the delay time of the delay circuit 116 as T, there obtain the following relations: ##EQU1##
The sin term of Formula (1) and the cos term of Formula (2) above represent the frequency characteristics of this phase shifting circuit. The exponential terms of Formulas (1) and (2) represent the phasic characteristics of the circuit. As obvious from Formulas (1) and (2), the phase of the output Fx is shifted 90 degrees from that of the output Fy.
FIG. 5(a) shows the frequency-amplitude characteristics of the outputs Fx and Fy. The delay time T of the delay circuit 116 is set at such a value that causes the center frequency of the frequency modulation carrier to be 1/4 T. FIG. 5(b) shows frequency modulation frequency allocation corresponding to FIG. 5(a).
The output Fx of the subtracter 118 is amplitude limited by the limiter 122. The output Fy of the adder 120 is amplitude limited by another limiter 124. The outputs of the limiters 122 and 124 are as schematically represented in FIG. 6(a) and 6(b). They are full-wave differentiated by the full-wave differentiation circuits 126 and 128. The outputs of the full-wave differentiation circuits 126 and 128 which are as shown in FIGS. 6(c) and 6(d) are applied to an OR circuit 130 to obtain a logical sum. The output of the OR gate 130 which is as shown in FIG. 6(e) is the 4-stepped-up signal of the input signal Fa of the demodulating device 100. The signal thus obtained is applied to the trigger input terminal of an MM 132. The MM 132 then generates a pulse train which is as shown in FIG. 6(f) and is supplied to an LPF 134. the LPF 134 then produces a demodulated signal.
The conventional 4-step-up demodulating device is thus arranged to use a 90 degree phase shifter of the transversal filter type for twice stepping up a frequency modulated wave. The transversal filter has a frequency characteristic which is not flat as shown in FIG. 5(a). Therefore, the conventional device presents the following problem: Since the outputs Fx and Fy differ from each other in frequency characteristic, the component of 2 fc remains without being canceled by the 4-step-up process. This component, therefore, appears as a moire on the picture plane in the case of a video signal. Further, since the outputs Fx and Fy have frequency characteristics, the demodulated signal also has a frequency characteristic, which causes the high band component of the signal to deteriorate. Although the filter characteristic can be arranged to be flat to solve that problem, such arrangement would result in an increase in manufacturing cost.